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Table of contents
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<li><a href="#modules-ecmascript-modules">Modules: ECMAScript modules</a>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#enabling">Enabling</a></li>
<li><a href="#packages">Packages</a></li>
<li><a href="#import-specifiers"><code>import</code> Specifiers</a>
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<li><a href="#terminology">Terminology</a></li>
<li><a href="#mandatory-file-extensions">Mandatory file extensions</a></li>
<li><a href="#urls">URLs</a>
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<li><a href="#file-urls"><code>file:</code> URLs</a></li>
<li><a href="#data-imports"><code>data:</code> imports</a></li>
<li><a href="#node-imports"><code>node:</code> imports</a></li>
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<li><span class="stability_1"><a href="#import-attributes">Import attributes</a></span></li>
<li><a href="#built-in-modules">Built-in modules</a></li>
<li><a href="#import-expressions"><code>import()</code> expressions</a></li>
<li><a href="#importmeta"><code>import.meta</code></a>
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<li><span class="stability_1"><a href="#importmetadirname"><code>import.meta.dirname</code></a></span></li>
<li><span class="stability_1"><a href="#importmetafilename"><code>import.meta.filename</code></a></span></li>
<li><a href="#importmetaurl"><code>import.meta.url</code></a></li>
<li><span class="stability_1"><a href="#importmetaresolvespecifier"><code>import.meta.resolve(specifier)</code></a></span></li>
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<li><a href="#interoperability-with-commonjs">Interoperability with CommonJS</a>
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<li><a href="#import-statements"><code>import</code> statements</a></li>
<li><a href="#require"><code>require</code></a></li>
<li><a href="#commonjs-namespaces">CommonJS Namespaces</a></li>
<li><a href="#differences-between-es-modules-and-commonjs">Differences between ES modules and CommonJS</a>
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<li><a href="#no-require-exports-or-moduleexports">No <code>require</code>, <code>exports</code>, or <code>module.exports</code></a></li>
<li><a href="#no-__filename-or-__dirname">No <code>__filename</code> or <code>__dirname</code></a></li>
<li><a href="#no-addon-loading">No Addon Loading</a></li>
<li><a href="#no-requireresolve">No <code>require.resolve</code></a></li>
<li><a href="#no-node_path">No <code>NODE_PATH</code></a></li>
<li><a href="#no-requireextensions">No <code>require.extensions</code></a></li>
<li><a href="#no-requirecache">No <code>require.cache</code></a></li>
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<li><span class="stability_1"><a href="#json-modules">JSON modules</a></span></li>
<li><span class="stability_1"><a href="#wasm-modules">Wasm modules</a></span></li>
<li><a href="#top-level-await">Top-level <code>await</code></a></li>
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<li><a href="https://nodejs.org/docs/latest-v12.x/api/esm.html">12.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v11.x/api/esm.html">11.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v10.x/api/esm.html">10.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v9.x/api/esm.html">9.x</a></li>
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<details role="navigation" id="toc" open><summary>Table of contents</summary><ul>
<li><a href="#modules-ecmascript-modules">Modules: ECMAScript modules</a>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#enabling">Enabling</a></li>
<li><a href="#packages">Packages</a></li>
<li><a href="#import-specifiers"><code>import</code> Specifiers</a>
<ul>
<li><a href="#terminology">Terminology</a></li>
<li><a href="#mandatory-file-extensions">Mandatory file extensions</a></li>
<li><a href="#urls">URLs</a>
<ul>
<li><a href="#file-urls"><code>file:</code> URLs</a></li>
<li><a href="#data-imports"><code>data:</code> imports</a></li>
<li><a href="#node-imports"><code>node:</code> imports</a></li>
</ul>
</li>
</ul>
</li>
<li><span class="stability_1"><a href="#import-attributes">Import attributes</a></span></li>
<li><a href="#built-in-modules">Built-in modules</a></li>
<li><a href="#import-expressions"><code>import()</code> expressions</a></li>
<li><a href="#importmeta"><code>import.meta</code></a>
<ul>
<li><span class="stability_1"><a href="#importmetadirname"><code>import.meta.dirname</code></a></span></li>
<li><span class="stability_1"><a href="#importmetafilename"><code>import.meta.filename</code></a></span></li>
<li><a href="#importmetaurl"><code>import.meta.url</code></a></li>
<li><span class="stability_1"><a href="#importmetaresolvespecifier"><code>import.meta.resolve(specifier)</code></a></span></li>
</ul>
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<li><a href="#interoperability-with-commonjs">Interoperability with CommonJS</a>
<ul>
<li><a href="#import-statements"><code>import</code> statements</a></li>
<li><a href="#require"><code>require</code></a></li>
<li><a href="#commonjs-namespaces">CommonJS Namespaces</a></li>
<li><a href="#differences-between-es-modules-and-commonjs">Differences between ES modules and CommonJS</a>
<ul>
<li><a href="#no-require-exports-or-moduleexports">No <code>require</code>, <code>exports</code>, or <code>module.exports</code></a></li>
<li><a href="#no-__filename-or-__dirname">No <code>__filename</code> or <code>__dirname</code></a></li>
<li><a href="#no-addon-loading">No Addon Loading</a></li>
<li><a href="#no-requireresolve">No <code>require.resolve</code></a></li>
<li><a href="#no-node_path">No <code>NODE_PATH</code></a></li>
<li><a href="#no-requireextensions">No <code>require.extensions</code></a></li>
<li><a href="#no-requirecache">No <code>require.cache</code></a></li>
</ul>
</li>
</ul>
</li>
<li><span class="stability_1"><a href="#json-modules">JSON modules</a></span></li>
<li><span class="stability_1"><a href="#wasm-modules">Wasm modules</a></span></li>
<li><a href="#top-level-await">Top-level <code>await</code></a></li>
<li><span class="stability_1"><a href="#https-and-http-imports">HTTPS and HTTP imports</a></span>
<ul>
<li><a href="#imports-are-limited-to-http1">Imports are limited to HTTP/1</a></li>
<li><a href="#http-is-limited-to-loopback-addresses">HTTP is limited to loopback addresses</a></li>
<li><a href="#authentication-is-never-sent-to-the-destination-server">Authentication is never sent to the destination server.</a></li>
<li><a href="#cors-is-never-checked-on-the-destination-server">CORS is never checked on the destination server</a></li>
<li><a href="#cannot-load-non-network-dependencies">Cannot load non-network dependencies</a></li>
<li><a href="#network-based-loading-is-not-enabled-by-default">Network-based loading is not enabled by default</a></li>
</ul>
</li>
<li><a href="#loaders">Loaders</a></li>
<li><a href="#resolution-and-loading-algorithm">Resolution and loading algorithm</a>
<ul>
<li><a href="#features">Features</a></li>
<li><a href="#resolution-algorithm">Resolution algorithm</a></li>
<li><a href="#resolution-algorithm-specification">Resolution Algorithm Specification</a></li>
<li><a href="#customizing-esm-specifier-resolution-algorithm">Customizing ESM specifier resolution algorithm</a></li>
</ul>
</li>
</ul>
</li>
</ul></details>
<div role="main" id="apicontent">
<h2>Modules: ECMAScript modules<span><a class="mark" href="#modules-ecmascript-modules" id="modules-ecmascript-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_modules_ecmascript_modules"></a></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.10.0</td>
<td><p>Add experimental support for import attributes.</p></td></tr>
<tr><td>v20.0.0</td>
<td><p>Module customization hooks are executed off the main thread.</p></td></tr>
<tr><td>v18.6.0, v16.17.0</td>
<td><p>Add support for chaining module customization hooks.</p></td></tr>
<tr><td>v17.1.0, v16.14.0</td>
<td><p>Add experimental support for import assertions.</p></td></tr>
<tr><td>v17.0.0, v16.12.0</td>
<td><p>Consolidate customization hooks, removed <code>getFormat</code>, <code>getSource</code>, <code>transformSource</code>, and <code>getGlobalPreloadCode</code> hooks added <code>load</code> and <code>globalPreload</code> hooks allowed returning <code>format</code> from either <code>resolve</code> or <code>load</code> hooks.</p></td></tr>
<tr><td>v14.8.0</td>
<td><p>Unflag Top-Level Await.</p></td></tr>
<tr><td>v15.3.0, v14.17.0, v12.22.0</td>
<td><p>Stabilize modules implementation.</p></td></tr>
<tr><td>v14.13.0, v12.20.0</td>
<td><p>Support for detection of CommonJS named exports.</p></td></tr>
<tr><td>v14.0.0, v13.14.0, v12.20.0</td>
<td><p>Remove experimental modules warning.</p></td></tr>
<tr><td>v13.2.0, v12.17.0</td>
<td><p>Loading ECMAScript modules no longer requires a command-line flag.</p></td></tr>
<tr><td>v12.0.0</td>
<td><p>Add support for ES modules using <code>.js</code> file extension via <code>package.json</code> <code>"type"</code> field.</p></td></tr>
<tr><td>v8.5.0</td>
<td><p><span>Added in: v8.5.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p>
<section><h3>Introduction<span><a class="mark" href="#introduction" id="introduction">#</a></span><a aria-hidden="true" class="legacy" id="esm_introduction"></a></h3>
<p>ECMAScript modules are <a href="https://tc39.github.io/ecma262/#sec-modules">the official standard format</a> to package JavaScript
code for reuse. Modules are defined using a variety of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import"><code>import</code></a> and
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export"><code>export</code></a> statements.</p>
<p>The following example of an ES module exports a function:</p>
<pre><code class="language-js"><span class="hljs-comment">// addTwo.mjs</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">addTwo</span>(<span class="hljs-params">num</span>) {
<span class="hljs-keyword">return</span> num + <span class="hljs-number">2</span>;
}
<span class="hljs-keyword">export</span> { addTwo };</code> <button class="copy-button">copy</button></pre>
<p>The following example of an ES module imports the function from <code>addTwo.mjs</code>:</p>
<pre><code class="language-js"><span class="hljs-comment">// app.mjs</span>
<span class="hljs-keyword">import</span> { addTwo } <span class="hljs-keyword">from</span> <span class="hljs-string">'./addTwo.mjs'</span>;
<span class="hljs-comment">// Prints: 6</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">addTwo</span>(<span class="hljs-number">4</span>));</code> <button class="copy-button">copy</button></pre>
<p>Node.js fully supports ECMAScript modules as they are currently specified and
provides interoperability between them and its original module format,
<a href="modules.html">CommonJS</a>.</p>
<!-- Anchors to make sure old links find a target -->
<p><i id="esm_package_json_type_field"></i><i id="esm_package_scope_and_file_extensions"></i><i id="esm_input_type_flag"></i></p>
</section><section><h3>Enabling<span><a class="mark" href="#enabling" id="enabling">#</a></span><a aria-hidden="true" class="legacy" id="esm_enabling"></a></h3>
<p>Node.js has two module systems: <a href="modules.html">CommonJS</a> modules and ECMAScript modules.</p>
<p>Authors can tell Node.js to interpret JavaScript as an ES module via the <code>.mjs</code>
file extension, the <code>package.json</code> <a href="packages.html#type"><code>"type"</code></a> field with a value <code>"module"</code>,
the <a href="cli.html#--input-typetype"><code>--input-type</code></a> flag with a value of <code>"module"</code>, or the
<a href="cli.html#--experimental-default-typetype"><code>--experimental-default-type</code></a> flag with a value of <code>"module"</code>. These are
explicit markers of code being intended to run as an ES module.</p>
<p>Inversely, authors can tell Node.js to interpret JavaScript as CommonJS via the
<code>.cjs</code> file extension, the <code>package.json</code> <a href="packages.html#type"><code>"type"</code></a> field with a value
<code>"commonjs"</code>, the <a href="cli.html#--input-typetype"><code>--input-type</code></a> flag with a value of <code>"commonjs"</code>, or the
<a href="cli.html#--experimental-default-typetype"><code>--experimental-default-type</code></a> flag with a value of <code>"commonjs"</code>.</p>
<p>When code lacks explicit markers for either module system, Node.js will inspect
the source code of a module to look for ES module syntax. If such syntax is
found, Node.js will run the code as an ES module; otherwise it will run the
module as CommonJS. See <a href="packages.html#determining-module-system">Determining module system</a> for more details.</p>
<!-- Anchors to make sure old links find a target -->
<p><i id="esm_package_entry_points"></i><i id="esm_main_entry_point_export"></i><i id="esm_subpath_exports"></i><i id="esm_package_exports_fallbacks"></i><i id="esm_exports_sugar"></i><i id="esm_conditional_exports"></i><i id="esm_nested_conditions"></i><i id="esm_self_referencing_a_package_using_its_name"></i><i id="esm_internal_package_imports"></i><i id="esm_dual_commonjs_es_module_packages"></i><i id="esm_dual_package_hazard"></i><i id="esm_writing_dual_packages_while_avoiding_or_minimizing_hazards"></i><i id="esm_approach_1_use_an_es_module_wrapper"></i><i id="esm_approach_2_isolate_state"></i></p>
</section><section><h3>Packages<span><a class="mark" href="#packages" id="packages">#</a></span><a aria-hidden="true" class="legacy" id="esm_packages"></a></h3>
<p>This section was moved to <a href="packages.html">Modules: Packages</a>.</p>
</section><section><h3><code>import</code> Specifiers<span><a class="mark" href="#import-specifiers" id="import-specifiers">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_specifiers"></a></h3>
<h4>Terminology<span><a class="mark" href="#terminology" id="terminology">#</a></span><a aria-hidden="true" class="legacy" id="esm_terminology"></a></h4>
<p>The <em>specifier</em> of an <code>import</code> statement is the string after the <code>from</code> keyword,
e.g. <code>'node:path'</code> in <code>import { sep } from 'node:path'</code>. Specifiers are also
used in <code>export from</code> statements, and as the argument to an <code>import()</code>
expression.</p>
<p>There are three types of specifiers:</p>
<ul>
<li>
<p><em>Relative specifiers</em> like <code>'./startup.js'</code> or <code>'../config.mjs'</code>. They refer
to a path relative to the location of the importing file. <em>The file extension
is always necessary for these.</em></p>
</li>
<li>
<p><em>Bare specifiers</em> like <code>'some-package'</code> or <code>'some-package/shuffle'</code>. They can
refer to the main entry point of a package by the package name, or a
specific feature module within a package prefixed by the package name as per
the examples respectively. <em>Including the file extension is only necessary
for packages without an <a href="packages.html#exports"><code>"exports"</code></a> field.</em></p>
</li>
<li>
<p><em>Absolute specifiers</em> like <code>'file:///opt/nodejs/config.js'</code>. They refer
directly and explicitly to a full path.</p>
</li>
</ul>
<p>Bare specifier resolutions are handled by the <a href="#resolution-algorithm-specification">Node.js module
resolution and loading algorithm</a>.
All other specifier resolutions are always only resolved with
the standard relative <a href="https://url.spec.whatwg.org/">URL</a> resolution semantics.</p>
<p>Like in CommonJS, module files within packages can be accessed by appending a
path to the package name unless the package's <a href="packages.html#nodejs-packagejson-field-definitions"><code>package.json</code></a> contains an
<a href="packages.html#exports"><code>"exports"</code></a> field, in which case files within packages can only be accessed
via the paths defined in <a href="packages.html#exports"><code>"exports"</code></a>.</p>
<p>For details on these package resolution rules that apply to bare specifiers in
the Node.js module resolution, see the <a href="packages.html">packages documentation</a>.</p>
<h4>Mandatory file extensions<span><a class="mark" href="#mandatory-file-extensions" id="mandatory-file-extensions">#</a></span><a aria-hidden="true" class="legacy" id="esm_mandatory_file_extensions"></a></h4>
<p>A file extension must be provided when using the <code>import</code> keyword to resolve
relative or absolute specifiers. Directory indexes (e.g. <code>'./startup/index.js'</code>)
must also be fully specified.</p>
<p>This behavior matches how <code>import</code> behaves in browser environments, assuming a
typically configured server.</p>
<h4>URLs<span><a class="mark" href="#urls" id="urls">#</a></span><a aria-hidden="true" class="legacy" id="esm_urls"></a></h4>
<p>ES modules are resolved and cached as URLs. This means that special characters
must be <a href="url.html#percent-encoding-in-urls">percent-encoded</a>, such as <code>#</code> with <code>%23</code> and <code>?</code> with <code>%3F</code>.</p>
<p><code>file:</code>, <code>node:</code>, and <code>data:</code> URL schemes are supported. A specifier like
<code>'https://example.com/app.js'</code> is not supported natively in Node.js unless using
a <a href="module.html#import-from-https">custom HTTPS loader</a>.</p>
<h5><code>file:</code> URLs<span><a class="mark" href="#file-urls" id="file-urls">#</a></span><a aria-hidden="true" class="legacy" id="esm_file_urls"></a></h5>
<p>Modules are loaded multiple times if the <code>import</code> specifier used to resolve
them has a different query or fragment.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-string">'./foo.mjs?query=1'</span>; <span class="hljs-comment">// loads ./foo.mjs with query of "?query=1"</span>
<span class="hljs-keyword">import</span> <span class="hljs-string">'./foo.mjs?query=2'</span>; <span class="hljs-comment">// loads ./foo.mjs with query of "?query=2"</span></code> <button class="copy-button">copy</button></pre>
<p>The volume root may be referenced via <code>/</code>, <code>//</code>, or <code>file:///</code>. Given the
differences between <a href="https://url.spec.whatwg.org/">URL</a> and path resolution (such as percent encoding
details), it is recommended to use <a href="url.html#urlpathtofileurlpath-options">url.pathToFileURL</a> when importing a path.</p>
<h5><code>data:</code> imports<span><a class="mark" href="#data-imports" id="data-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_data_imports"></a></h5>
<div class="api_metadata">
<span>Added in: v12.10.0</span>
</div>
<p><a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs"><code>data:</code> URLs</a> are supported for importing with the following MIME types:</p>
<ul>
<li><code>text/javascript</code> for ES modules</li>
<li><code>application/json</code> for JSON</li>
<li><code>application/wasm</code> for Wasm</li>
</ul>
<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-string">'data:text/javascript,console.log("hello!");'</span>;
<span class="hljs-keyword">import</span> _ <span class="hljs-keyword">from</span> <span class="hljs-string">'data:application/json,"world!"'</span> <span class="hljs-keyword">with</span> { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> };</code> <button class="copy-button">copy</button></pre>
<p><code>data:</code> URLs only resolve <a href="#terminology">bare specifiers</a> for builtin modules
and <a href="#terminology">absolute specifiers</a>. Resolving
<a href="#terminology">relative specifiers</a> does not work because <code>data:</code> is not a
<a href="https://url.spec.whatwg.org/#special-scheme">special scheme</a>. For example, attempting to load <code>./foo</code>
from <code>data:text/javascript,import "./foo";</code> fails to resolve because there
is no concept of relative resolution for <code>data:</code> URLs.</p>
<h5><code>node:</code> imports<span><a class="mark" href="#node-imports" id="node-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_node_imports"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.0.0, v14.18.0</td>
<td><p>Added <code>node:</code> import support to <code>require(...)</code>.</p></td></tr>
<tr><td>v14.13.1, v12.20.0</td>
<td><p><span>Added in: v14.13.1, v12.20.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p><code>node:</code> URLs are supported as an alternative means to load Node.js builtin
modules. This URL scheme allows for builtin modules to be referenced by valid
absolute URL strings.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>;</code> <button class="copy-button">copy</button></pre>
<p><a id="import-assertions"></a></p>
</section><section><h3>Import attributes<span><a class="mark" href="#import-attributes" id="import-attributes">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_attributes"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.10.0</td>
<td><p>Switch from Import Assertions to Import Attributes.</p></td></tr>
<tr><td>v17.1.0, v16.14.0</td>
<td><p><span>Added in: v17.1.0, v16.14.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.1 - Active development</div><p></p>
<blockquote>
<p>This feature was previously named "Import assertions", and using the <code>assert</code>
keyword instead of <code>with</code>. Any uses in code of the prior <code>assert</code> keyword
should be updated to use <code>with</code> instead.</p>
</blockquote>
<p>The <a href="https://github.com/tc39/proposal-import-attributes">Import Attributes proposal</a> adds an inline syntax for module import
statements to pass on more information alongside the module specifier.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> fooData <span class="hljs-keyword">from</span> <span class="hljs-string">'./foo.json'</span> <span class="hljs-keyword">with</span> { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> };
<span class="hljs-keyword">const</span> { <span class="hljs-attr">default</span>: barData } =
<span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>(<span class="hljs-string">'./bar.json'</span>, { <span class="hljs-attr">with</span>: { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> } });</code> <button class="copy-button">copy</button></pre>
<p>Node.js supports the following <code>type</code> values, for which the attribute is
mandatory:</p>
<table><thead><tr><th>Attribute <code>type</code></th><th>Needed for</th></tr></thead><tbody><tr><td><code>'json'</code></td><td><a href="#json-modules">JSON modules</a></td></tr></tbody></table>
</section><section><h3>Built-in modules<span><a class="mark" href="#built-in-modules" id="built-in-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_built_in_modules"></a></h3>
<p><a href="modules.html#built-in-modules">Built-in modules</a> provide named exports of their public API. A
default export is also provided which is the value of the CommonJS exports.
The default export can be used for, among other things, modifying the named
exports. Named exports of built-in modules are updated only by calling
<a href="module.html#modulesyncbuiltinesmexports"><code>module.syncBuiltinESMExports()</code></a>.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> <span class="hljs-title class_">EventEmitter</span> <span class="hljs-keyword">from</span> <span class="hljs-string">'node:events'</span>;
<span class="hljs-keyword">const</span> e = <span class="hljs-keyword">new</span> <span class="hljs-title class_">EventEmitter</span>();</code> <button class="copy-button">copy</button></pre>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-title function_">readFile</span>(<span class="hljs-string">'./foo.txt'</span>, <span class="hljs-function">(<span class="hljs-params">err, source</span>) =></span> {
<span class="hljs-keyword">if</span> (err) {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(err);
} <span class="hljs-keyword">else</span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(source);
}
});</code> <button class="copy-button">copy</button></pre>
<pre><code class="language-js"><span class="hljs-keyword">import</span> fs, { readFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-keyword">import</span> { syncBuiltinESMExports } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">Buffer</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:buffer'</span>;
fs.<span class="hljs-property">readFileSync</span> = <span class="hljs-function">() =></span> <span class="hljs-title class_">Buffer</span>.<span class="hljs-title function_">from</span>(<span class="hljs-string">'Hello, ESM'</span>);
<span class="hljs-title function_">syncBuiltinESMExports</span>();
fs.<span class="hljs-property">readFileSync</span> === readFileSync;</code> <button class="copy-button">copy</button></pre>
</section><section><h3><code>import()</code> expressions<span><a class="mark" href="#import-expressions" id="import-expressions">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_expressions"></a></h3>
<p><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import">Dynamic <code>import()</code></a> is supported in both CommonJS and ES modules. In CommonJS
modules it can be used to load ES modules.</p>
</section><section><h3><code>import.meta</code><span><a class="mark" href="#importmeta" id="importmeta">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta"></a></h3>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
<p>The <code>import.meta</code> meta property is an <code>Object</code> that contains the following
properties.</p>
<h4><code>import.meta.dirname</code><span><a class="mark" href="#importmetadirname" id="importmetadirname">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_dirname"></a></h4>
<div class="api_metadata">
<span>Added in: v20.11.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The directory name of the current module. This is the same as the
<a href="path.html#pathdirnamepath"><code>path.dirname()</code></a> of the <a href="#importmetafilename"><code>import.meta.filename</code></a>.</li>
</ul>
<blockquote>
<p><strong>Caveat</strong>: only present on <code>file:</code> modules.</p>
</blockquote>
<h4><code>import.meta.filename</code><span><a class="mark" href="#importmetafilename" id="importmetafilename">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_filename"></a></h4>
<div class="api_metadata">
<span>Added in: v20.11.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The full absolute path and filename of the current module, with
symlinks resolved.</li>
<li>This is the same as the <a href="url.html#urlfileurltopathurl-options"><code>url.fileURLToPath()</code></a> of the
<a href="#importmetaurl"><code>import.meta.url</code></a>.</li>
</ul>
<blockquote>
<p><strong>Caveat</strong> only local modules support this property. Modules not using the
<code>file:</code> protocol will not provide it.</p>
</blockquote>
<h4><code>import.meta.url</code><span><a class="mark" href="#importmetaurl" id="importmetaurl">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_url"></a></h4>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The absolute <code>file:</code> URL of the module.</li>
</ul>
<p>This is defined exactly the same as it is in browsers providing the URL of the
current module file.</p>
<p>This enables useful patterns such as relative file loading:</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { readFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-keyword">const</span> buffer = <span class="hljs-title function_">readFileSync</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-string">'./data.proto'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>));</code> <button class="copy-button">copy</button></pre>
<h4><code>import.meta.resolve(specifier)</code><span><a class="mark" href="#importmetaresolvespecifier" id="importmetaresolvespecifier">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_meta_resolve_specifier"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.6.0</td>
<td><p>Unflag <code>import.meta.resolve</code>, with <code>parentURL</code> parameter still flagged.</p></td></tr>
<tr><td>v20.6.0</td>
<td><p>This API no longer throws when targeting <code>file:</code> URLs that do not map to an existing file on the local FS.</p></td></tr>
<tr><td>v20.0.0</td>
<td><p>This API now returns a string synchronously instead of a Promise.</p></td></tr>
<tr><td>v16.2.0, v14.18.0</td>
<td><p>Add support for WHATWG <code>URL</code> object to <code>parentURL</code> parameter.</p></td></tr>
<tr><td>v13.9.0, v12.16.2</td>
<td><p><span>Added in: v13.9.0, v12.16.2</span></p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<ul>
<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The module specifier to resolve relative to the
current module.</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The absolute URL string that the specifier would resolve to.</li>
</ul>
<p><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import.meta/resolve"><code>import.meta.resolve</code></a> is a module-relative resolution function scoped to
each module, returning the URL string.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> dependencyAsset = <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-string">'component-lib/asset.css'</span>);
<span class="hljs-comment">// file:///app/node_modules/component-lib/asset.css</span>
<span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-string">'./dep.js'</span>);
<span class="hljs-comment">// file:///app/dep.js</span></code> <button class="copy-button">copy</button></pre>
<p>All features of the Node.js module resolution are supported. Dependency
resolutions are subject to the permitted exports resolutions within the package.</p>
<p><strong>Caveats</strong>:</p>
<ul>
<li>This can result in synchronous file-system operations, which
can impact performance similarly to <code>require.resolve</code>.</li>
<li>This feature is not available within custom loaders (it would
create a deadlock).</li>
</ul>
<p><strong>Non-standard API</strong>:</p>
<p>When using the <code>--experimental-import-meta-resolve</code> flag, that function accepts
a second argument:</p>
<ul>
<li><code>parent</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> An optional absolute parent module URL to resolve from.
<strong>Default:</strong> <code>import.meta.url</code></li>
</ul>
</section><section><h3>Interoperability with CommonJS<span><a class="mark" href="#interoperability-with-commonjs" id="interoperability-with-commonjs">#</a></span><a aria-hidden="true" class="legacy" id="esm_interoperability_with_commonjs"></a></h3>
<h4><code>import</code> statements<span><a class="mark" href="#import-statements" id="import-statements">#</a></span><a aria-hidden="true" class="legacy" id="esm_import_statements"></a></h4>
<p>An <code>import</code> statement can reference an ES module or a CommonJS module.
<code>import</code> statements are permitted only in ES modules, but dynamic <a href="#import-expressions"><code>import()</code></a>
expressions are supported in CommonJS for loading ES modules.</p>
<p>When importing <a href="#commonjs-namespaces">CommonJS modules</a>, the
<code>module.exports</code> object is provided as the default export. Named exports may be
available, provided by static analysis as a convenience for better ecosystem
compatibility.</p>
<h4><code>require</code><span><a class="mark" href="#require" id="require">#</a></span><a aria-hidden="true" class="legacy" id="esm_require"></a></h4>
<p>The CommonJS module <code>require</code> currently only supports loading synchronous ES
modules when <code>--experimental-require-module</code> is enabled.</p>
<p>See <a href="modules.html#loading-ecmascript-modules-using-require">Loading ECMAScript modules using <code>require()</code></a> for details.</p>
<h4>CommonJS Namespaces<span><a class="mark" href="#commonjs-namespaces" id="commonjs-namespaces">#</a></span><a aria-hidden="true" class="legacy" id="esm_commonjs_namespaces"></a></h4>
<p>CommonJS modules consist of a <code>module.exports</code> object which can be of any type.</p>
<p>When importing a CommonJS module, it can be reliably imported using the ES
module default import or its corresponding sugar syntax:</p>
<!-- eslint-disable no-duplicate-imports -->
<pre><code class="language-js"><span class="hljs-keyword">import</span> { <span class="hljs-keyword">default</span> <span class="hljs-keyword">as</span> cjs } <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>;
<span class="hljs-comment">// The following import statement is "syntax sugar" (equivalent but sweeter)</span>
<span class="hljs-comment">// for `{ default as cjsSugar }` in the above import statement:</span>
<span class="hljs-keyword">import</span> cjsSugar <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs === cjsSugar);
<span class="hljs-comment">// Prints:</span>
<span class="hljs-comment">// <module.exports></span>
<span class="hljs-comment">// true</span></code> <button class="copy-button">copy</button></pre>
<p>The ECMAScript Module Namespace representation of a CommonJS module is always
a namespace with a <code>default</code> export key pointing to the CommonJS
<code>module.exports</code> value.</p>
<p>This Module Namespace Exotic Object can be directly observed either when using
<code>import * as m from 'cjs'</code> or a dynamic import:</p>
<!-- eslint-skip -->
<pre><code class="language-js"><span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> m <span class="hljs-keyword">from</span> <span class="hljs-string">'cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m === <span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>(<span class="hljs-string">'cjs'</span>));
<span class="hljs-comment">// Prints:</span>
<span class="hljs-comment">// [Module] { default: <module.exports> }</span>
<span class="hljs-comment">// true</span></code> <button class="copy-button">copy</button></pre>
<p>For better compatibility with existing usage in the JS ecosystem, Node.js
in addition attempts to determine the CommonJS named exports of every imported
CommonJS module to provide them as separate ES module exports using a static
analysis process.</p>
<p>For example, consider a CommonJS module written:</p>
<pre><code class="language-js cjs"><span class="hljs-comment">// cjs.cjs</span>
<span class="hljs-built_in">exports</span>.<span class="hljs-property">name</span> = <span class="hljs-string">'exported'</span>;</code> <button class="copy-button">copy</button></pre>
<p>The preceding module supports named imports in ES modules:</p>
<!-- eslint-disable no-duplicate-imports -->
<pre><code class="language-js"><span class="hljs-keyword">import</span> { name } <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(name);
<span class="hljs-comment">// Prints: 'exported'</span>
<span class="hljs-keyword">import</span> cjs <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(cjs);
<span class="hljs-comment">// Prints: { name: 'exported' }</span>
<span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> m <span class="hljs-keyword">from</span> <span class="hljs-string">'./cjs.cjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(m);
<span class="hljs-comment">// Prints: [Module] { default: { name: 'exported' }, name: 'exported' }</span></code> <button class="copy-button">copy</button></pre>
<p>As can be seen from the last example of the Module Namespace Exotic Object being
logged, the <code>name</code> export is copied off of the <code>module.exports</code> object and set
directly on the ES module namespace when the module is imported.</p>
<p>Live binding updates or new exports added to <code>module.exports</code> are not detected
for these named exports.</p>
<p>The detection of named exports is based on common syntax patterns but does not
always correctly detect named exports. In these cases, using the default
import form described above can be a better option.</p>
<p>Named exports detection covers many common export patterns, reexport patterns
and build tool and transpiler outputs. See <a href="https://github.com/nodejs/cjs-module-lexer/tree/1.2.2">cjs-module-lexer</a> for the exact
semantics implemented.</p>
<h4>Differences between ES modules and CommonJS<span><a class="mark" href="#differences-between-es-modules-and-commonjs" id="differences-between-es-modules-and-commonjs">#</a></span><a aria-hidden="true" class="legacy" id="esm_differences_between_es_modules_and_commonjs"></a></h4>
<h5>No <code>require</code>, <code>exports</code>, or <code>module.exports</code><span><a class="mark" href="#no-require-exports-or-moduleexports" id="no-require-exports-or-moduleexports">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_exports_or_module_exports"></a></h5>
<p>In most cases, the ES module <code>import</code> can be used to load CommonJS modules.</p>
<p>If needed, a <code>require</code> function can be constructed within an ES module using
<a href="module.html#modulecreaterequirefilename"><code>module.createRequire()</code></a>.</p>
<h5>No <code>__filename</code> or <code>__dirname</code><span><a class="mark" href="#no-__filename-or-__dirname" id="no-__filename-or-__dirname">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_filename_or_dirname"></a></h5>
<p>These CommonJS variables are not available in ES modules.</p>
<p><code>__filename</code> and <code>__dirname</code> use cases can be replicated via
<a href="#importmetafilename"><code>import.meta.filename</code></a> and <a href="#importmetadirname"><code>import.meta.dirname</code></a>.</p>
<h5>No Addon Loading<span><a class="mark" href="#no-addon-loading" id="no-addon-loading">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_addon_loading"></a></h5>
<p><a href="addons.html">Addons</a> are not currently supported with ES module imports.</p>
<p>They can instead be loaded with <a href="module.html#modulecreaterequirefilename"><code>module.createRequire()</code></a> or
<a href="process.html#processdlopenmodule-filename-flags"><code>process.dlopen</code></a>.</p>
<h5>No <code>require.resolve</code><span><a class="mark" href="#no-requireresolve" id="no-requireresolve">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_resolve"></a></h5>
<p>Relative resolution can be handled via <code>new URL('./local', import.meta.url)</code>.</p>
<p>For a complete <code>require.resolve</code> replacement, there is the
<a href="#importmetaresolvespecifier">import.meta.resolve</a> API.</p>
<p>Alternatively <code>module.createRequire()</code> can be used.</p>
<h5>No <code>NODE_PATH</code><span><a class="mark" href="#no-node_path" id="no-node_path">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_node_path"></a></h5>
<p><code>NODE_PATH</code> is not part of resolving <code>import</code> specifiers. Please use symlinks
if this behavior is desired.</p>
<h5>No <code>require.extensions</code><span><a class="mark" href="#no-requireextensions" id="no-requireextensions">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_extensions"></a></h5>
<p><code>require.extensions</code> is not used by <code>import</code>. Module customization hooks can
provide a replacement.</p>
<h5>No <code>require.cache</code><span><a class="mark" href="#no-requirecache" id="no-requirecache">#</a></span><a aria-hidden="true" class="legacy" id="esm_no_require_cache"></a></h5>
<p><code>require.cache</code> is not used by <code>import</code> as the ES module loader has its own
separate cache.</p>
<p><i id="esm_experimental_json_modules"></i></p>
</section><section><h3>JSON modules<span><a class="mark" href="#json-modules" id="json-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_json_modules"></a></h3>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>JSON files can be referenced by <code>import</code>:</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> packageConfig <span class="hljs-keyword">from</span> <span class="hljs-string">'./package.json'</span> <span class="hljs-keyword">with</span> { <span class="hljs-attr">type</span>: <span class="hljs-string">'json'</span> };</code> <button class="copy-button">copy</button></pre>
<p>The <code>with { type: 'json' }</code> syntax is mandatory; see <a href="#import-attributes">Import Attributes</a>.</p>
<p>The imported JSON only exposes a <code>default</code> export. There is no support for named
exports. A cache entry is created in the CommonJS cache to avoid duplication.
The same object is returned in CommonJS if the JSON module has already been
imported from the same path.</p>
<p><i id="esm_experimental_wasm_modules"></i></p>
</section><section><h3>Wasm modules<span><a class="mark" href="#wasm-modules" id="wasm-modules">#</a></span><a aria-hidden="true" class="legacy" id="esm_wasm_modules"></a></h3>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>Importing WebAssembly modules is supported under the
<code>--experimental-wasm-modules</code> flag, allowing any <code>.wasm</code> files to be
imported as normal modules while also supporting their module imports.</p>
<p>This integration is in line with the
<a href="https://github.com/webassembly/esm-integration">ES Module Integration Proposal for WebAssembly</a>.</p>
<p>For example, an <code>index.mjs</code> containing:</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> * <span class="hljs-keyword">as</span> M <span class="hljs-keyword">from</span> <span class="hljs-string">'./module.wasm'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(M);</code> <button class="copy-button">copy</button></pre>
<p>executed under:</p>
<pre><code class="language-bash">node --experimental-wasm-modules index.mjs</code> <button class="copy-button">copy</button></pre>
<p>would provide the exports interface for the instantiation of <code>module.wasm</code>.</p>
<p><i id="esm_experimental_top_level_await"></i></p>
</section><section><h3>Top-level <code>await</code><span><a class="mark" href="#top-level-await" id="top-level-await">#</a></span><a aria-hidden="true" class="legacy" id="esm_top_level_await"></a></h3>
<div class="api_metadata">
<span>Added in: v14.8.0</span>
</div>
<p>The <code>await</code> keyword may be used in the top level body of an ECMAScript module.</p>
<p>Assuming an <code>a.mjs</code> with</p>
<pre><code class="language-js"><span class="hljs-keyword">export</span> <span class="hljs-keyword">const</span> five = <span class="hljs-keyword">await</span> <span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">5</span>);</code> <button class="copy-button">copy</button></pre>
<p>And a <code>b.mjs</code> with</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { five } <span class="hljs-keyword">from</span> <span class="hljs-string">'./a.mjs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(five); <span class="hljs-comment">// Logs `5`</span></code> <button class="copy-button">copy</button></pre>
<pre><code class="language-bash">node b.mjs <span class="hljs-comment"># works</span></code> <button class="copy-button">copy</button></pre>
<p>If a top level <code>await</code> expression never resolves, the <code>node</code> process will exit
with a <code>13</code> <a href="process.html#exit-codes">status code</a>.</p>
<pre><code class="language-js"><span class="hljs-keyword">import</span> { spawn } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:child_process'</span>;
<span class="hljs-keyword">import</span> { execPath } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>;
<span class="hljs-title function_">spawn</span>(execPath, [
<span class="hljs-string">'--input-type=module'</span>,
<span class="hljs-string">'--eval'</span>,
<span class="hljs-comment">// Never-resolving Promise:</span>
<span class="hljs-string">'await new Promise(() => {})'</span>,
]).<span class="hljs-title function_">once</span>(<span class="hljs-string">'exit'</span>, <span class="hljs-function">(<span class="hljs-params">code</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(code); <span class="hljs-comment">// Logs `13`</span>
});</code> <button class="copy-button">copy</button></pre>
</section><section><h3>HTTPS and HTTP imports<span><a class="mark" href="#https-and-http-imports" id="https-and-http-imports">#</a></span><a aria-hidden="true" class="legacy" id="esm_https_and_http_imports"></a></h3>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>Importing network based modules using <code>https:</code> and <code>http:</code> is supported under
the <code>--experimental-network-imports</code> flag. This allows web browser-like imports
to work in Node.js with a few differences due to application stability and
security concerns that are different when running in a privileged environment
instead of a browser sandbox.</p>
<h4>Imports are limited to HTTP/1<span><a class="mark" href="#imports-are-limited-to-http1" id="imports-are-limited-to-http1">#</a></span><a aria-hidden="true" class="legacy" id="esm_imports_are_limited_to_http_1"></a></h4>
<p>Automatic protocol negotiation for HTTP/2 and HTTP/3 is not yet supported.</p>
<h4>HTTP is limited to loopback addresses<span><a class="mark" href="#http-is-limited-to-loopback-addresses" id="http-is-limited-to-loopback-addresses">#</a></span><a aria-hidden="true" class="legacy" id="esm_http_is_limited_to_loopback_addresses"></a></h4>
<p><code>http:</code> is vulnerable to man-in-the-middle attacks and is not allowed to be
used for addresses outside of the IPv4 address <code>127.0.0.0/8</code> (<code>127.0.0.1</code> to
<code>127.255.255.255</code>) and the IPv6 address <code>::1</code>. Support for <code>http:</code> is intended
to be used for local development.</p>
<h4>Authentication is never sent to the destination server.<span><a class="mark" href="#authentication-is-never-sent-to-the-destination-server" id="authentication-is-never-sent-to-the-destination-server">#</a></span><a aria-hidden="true" class="legacy" id="esm_authentication_is_never_sent_to_the_destination_server"></a></h4>
<p><code>Authorization</code>, <code>Cookie</code>, and <code>Proxy-Authorization</code> headers are not sent to the
server. Avoid including user info in parts of imported URLs. A security model
for safely using these on the server is being worked on.</p>
<h4>CORS is never checked on the destination server<span><a class="mark" href="#cors-is-never-checked-on-the-destination-server" id="cors-is-never-checked-on-the-destination-server">#</a></span><a aria-hidden="true" class="legacy" id="esm_cors_is_never_checked_on_the_destination_server"></a></h4>
<p>CORS is designed to allow a server to limit the consumers of an API to a
specific set of hosts. This is not supported as it does not make sense for a
server-based implementation.</p>
<h4>Cannot load non-network dependencies<span><a class="mark" href="#cannot-load-non-network-dependencies" id="cannot-load-non-network-dependencies">#</a></span><a aria-hidden="true" class="legacy" id="esm_cannot_load_non_network_dependencies"></a></h4>
<p>These modules cannot access other modules that are not over <code>http:</code> or <code>https:</code>.
To still access local modules while avoiding the security concern, pass in
references to the local dependencies:</p>
<pre><code class="language-js mjs"><span class="hljs-comment">// file.mjs</span>
<span class="hljs-keyword">import</span> worker_threads <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>;
<span class="hljs-keyword">import</span> { configure, resize } <span class="hljs-keyword">from</span> <span class="hljs-string">'https://example.com/imagelib.mjs'</span>;
<span class="hljs-title function_">configure</span>({ worker_threads });</code> <button class="copy-button">copy</button></pre>
<pre><code class="language-js mjs"><span class="hljs-comment">// https://example.com/imagelib.mjs</span>
<span class="hljs-keyword">let</span> worker_threads;
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">configure</span>(<span class="hljs-params">opts</span>) {
worker_threads = opts.<span class="hljs-property">worker_threads</span>;
}
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resize</span>(<span class="hljs-params">img, size</span>) {
<span class="hljs-comment">// Perform resizing in worker_thread to avoid main thread blocking</span>
}</code> <button class="copy-button">copy</button></pre>
<h4>Network-based loading is not enabled by default<span><a class="mark" href="#network-based-loading-is-not-enabled-by-default" id="network-based-loading-is-not-enabled-by-default">#</a></span><a aria-hidden="true" class="legacy" id="esm_network_based_loading_is_not_enabled_by_default"></a></h4>
<p>For now, the <code>--experimental-network-imports</code> flag is required to enable loading
resources over <code>http:</code> or <code>https:</code>. In the future, a different mechanism will be
used to enforce this. Opt-in is required to prevent transitive dependencies
inadvertently using potentially mutable state that could affect reliability
of Node.js applications.</p>
<p><i id="esm_experimental_loaders"></i></p>
</section><section><h3>Loaders<span><a class="mark" href="#loaders" id="loaders">#</a></span><a aria-hidden="true" class="legacy" id="esm_loaders"></a></h3>
<p>The former Loaders documentation is now at
<a href="module.html#customization-hooks">Modules: Customization hooks</a>.</p>
</section><section><h3>Resolution and loading algorithm<span><a class="mark" href="#resolution-and-loading-algorithm" id="resolution-and-loading-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolution_and_loading_algorithm"></a></h3>
<h4>Features<span><a class="mark" href="#features" id="features">#</a></span><a aria-hidden="true" class="legacy" id="esm_features"></a></h4>
<p>The default resolver has the following properties:</p>
<ul>
<li>FileURL-based resolution as is used by ES modules</li>
<li>Relative and absolute URL resolution</li>
<li>No default extensions</li>
<li>No folder mains</li>
<li>Bare specifier package resolution lookup through node_modules</li>
<li>Does not fail on unknown extensions or protocols</li>
<li>Can optionally provide a hint of the format to the loading phase</li>
</ul>
<p>The default loader has the following properties</p>
<ul>
<li>Support for builtin module loading via <code>node:</code> URLs</li>
<li>Support for "inline" module loading via <code>data:</code> URLs</li>
<li>Support for <code>file:</code> module loading</li>
<li>Fails on any other URL protocol</li>
<li>Fails on unknown extensions for <code>file:</code> loading
(supports only <code>.cjs</code>, <code>.js</code>, and <code>.mjs</code>)</li>
</ul>
<h4>Resolution algorithm<span><a class="mark" href="#resolution-algorithm" id="resolution-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolution_algorithm"></a></h4>
<p>The algorithm to load an ES module specifier is given through the
<strong>ESM_RESOLVE</strong> method below. It returns the resolved URL for a
module specifier relative to a parentURL.</p>
<p>The resolution algorithm determines the full resolved URL for a module
load, along with its suggested module format. The resolution algorithm
does not determine whether the resolved URL protocol can be loaded,
or whether the file extensions are permitted, instead these validations
are applied by Node.js during the load phase
(for example, if it was asked to load a URL that has a protocol that is
not <code>file:</code>, <code>data:</code>, <code>node:</code>, or if <code>--experimental-network-imports</code>
is enabled, <code>https:</code>).</p>
<p>The algorithm also tries to determine the format of the file based
on the extension (see <code>ESM_FILE_FORMAT</code> algorithm below). If it does
not recognize the file extension (eg if it is not <code>.mjs</code>, <code>.cjs</code>, or
<code>.json</code>), then a format of <code>undefined</code> is returned,
which will throw during the load phase.</p>
<p>The algorithm to determine the module format of a resolved URL is
provided by <strong>ESM_FILE_FORMAT</strong>, which returns the unique module
format for any file. The <em>"module"</em> format is returned for an ECMAScript
Module, while the <em>"commonjs"</em> format is used to indicate loading through the
legacy CommonJS loader. Additional formats such as <em>"addon"</em> can be extended in
future updates.</p>
<p>In the following algorithms, all subroutine errors are propagated as errors
of these top-level routines unless stated otherwise.</p>
<p><em>defaultConditions</em> is the conditional environment name array,
<code>["node", "import"]</code>.</p>
<p>The resolver can throw the following errors:</p>
<ul>
<li><em>Invalid Module Specifier</em>: Module specifier is an invalid URL, package name
or package subpath specifier.</li>
<li><em>Invalid Package Configuration</em>: package.json configuration is invalid or
contains an invalid configuration.</li>
<li><em>Invalid Package Target</em>: Package exports or imports define a target module
for the package that is an invalid type or string target.</li>
<li><em>Package Path Not Exported</em>: Package exports do not define or permit a target
subpath in the package for the given module.</li>
<li><em>Package Import Not Defined</em>: Package imports do not define the specifier.</li>
<li><em>Module Not Found</em>: The package or module requested does not exist.</li>
<li><em>Unsupported Directory Import</em>: The resolved path corresponds to a directory,
which is not a supported target for module imports.</li>
</ul>
<h4>Resolution Algorithm Specification<span><a class="mark" href="#resolution-algorithm-specification" id="resolution-algorithm-specification">#</a></span><a aria-hidden="true" class="legacy" id="esm_resolution_algorithm_specification"></a></h4>
<p><strong>ESM_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>resolved</em> be <strong>undefined</strong>.</li>
<li>If <em>specifier</em> is a valid URL, then
<ol>
<li>Set <em>resolved</em> to the result of parsing and reserializing
<em>specifier</em> as a URL.</li>
</ol>
</li>
<li>Otherwise, if <em>specifier</em> starts with <em>"/"</em>, <em>"./"</em>, or <em>"../"</em>, then
<ol>
<li>Set <em>resolved</em> to the URL resolution of <em>specifier</em> relative to
<em>parentURL</em>.</li>
</ol>
</li>
<li>Otherwise, if <em>specifier</em> starts with <em>"#"</em>, then
<ol>
<li>Set <em>resolved</em> to the result of
<strong>PACKAGE_IMPORTS_RESOLVE</strong>(<em>specifier</em>,
<em>parentURL</em>, <em>defaultConditions</em>).</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Note: <em>specifier</em> is now a bare specifier.</li>
<li>Set <em>resolved</em> the result of
<strong>PACKAGE_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>).</li>
</ol>
</li>
<li>Let <em>format</em> be <strong>undefined</strong>.</li>
<li>If <em>resolved</em> is a <em>"file:"</em> URL, then
<ol>
<li>If <em>resolved</em> contains any percent encodings of <em>"/"</em> or <em>"\"</em> (<em>"%2F"</em>
and <em>"%5C"</em> respectively), then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>If the file at <em>resolved</em> is a directory, then
<ol>
<li>Throw an <em>Unsupported Directory Import</em> error.</li>
</ol>
</li>
<li>If the file at <em>resolved</em> does not exist, then
<ol>
<li>Throw a <em>Module Not Found</em> error.</li>
</ol>
</li>
<li>Set <em>resolved</em> to the real path of <em>resolved</em>, maintaining the
same URL querystring and fragment components.</li>
<li>Set <em>format</em> to the result of <strong>ESM_FILE_FORMAT</strong>(<em>resolved</em>).</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Set <em>format</em> the module format of the content type associated with the
URL <em>resolved</em>.</li>
</ol>
</li>
<li>Return <em>format</em> and <em>resolved</em> to the loading phase</li>
</ol>
</blockquote>
<p><strong>PACKAGE_RESOLVE</strong>(<em>packageSpecifier</em>, <em>parentURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>packageName</em> be <strong>undefined</strong>.</li>
<li>If <em>packageSpecifier</em> is an empty string, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>If <em>packageSpecifier</em> is a Node.js builtin module name, then
<ol>
<li>Return the string <em>"node:"</em> concatenated with <em>packageSpecifier</em>.</li>
</ol>
</li>
<li>If <em>packageSpecifier</em> does not start with <em>"@"</em>, then
<ol>
<li>Set <em>packageName</em> to the substring of <em>packageSpecifier</em> until the first
<em>"/"</em> separator or the end of the string.</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>If <em>packageSpecifier</em> does not contain a <em>"/"</em> separator, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Set <em>packageName</em> to the substring of <em>packageSpecifier</em>
until the second <em>"/"</em> separator or the end of the string.</li>
</ol>
</li>
<li>If <em>packageName</em> starts with <em>"."</em> or contains <em>"\"</em> or <em>"%"</em>, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Let <em>packageSubpath</em> be <em>"."</em> concatenated with the substring of
<em>packageSpecifier</em> from the position at the length of <em>packageName</em>.</li>
<li>If <em>packageSubpath</em> ends in <em>"/"</em>, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Let <em>selfUrl</em> be the result of
<strong>PACKAGE_SELF_RESOLVE</strong>(<em>packageName</em>, <em>packageSubpath</em>, <em>parentURL</em>).</li>
<li>If <em>selfUrl</em> is not <strong>undefined</strong>, return <em>selfUrl</em>.</li>
<li>While <em>parentURL</em> is not the file system root,
<ol>
<li>Let <em>packageURL</em> be the URL resolution of <em>"node_modules/"</em>
concatenated with <em>packageSpecifier</em>, relative to <em>parentURL</em>.</li>
<li>Set <em>parentURL</em> to the parent folder URL of <em>parentURL</em>.</li>
<li>If the folder at <em>packageURL</em> does not exist, then
<ol>
<li>Continue the next loop iteration.</li>
</ol>
</li>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>If <em>pjson</em> is not <strong>null</strong> and <em>pjson</em>.<em>exports</em> is not <strong>null</strong> or
<strong>undefined</strong>, then
<ol>
<li>Return the result of <strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>,
<em>packageSubpath</em>, <em>pjson.exports</em>, <em>defaultConditions</em>).</li>
</ol>
</li>
<li>Otherwise, if <em>packageSubpath</em> is equal to <em>"."</em>, then
<ol>
<li>If <em>pjson.main</em> is a string, then
<ol>
<li>Return the URL resolution of <em>main</em> in <em>packageURL</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Otherwise,
<ol>
<li>Return the URL resolution of <em>packageSubpath</em> in <em>packageURL</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Throw a <em>Module Not Found</em> error.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_SELF_RESOLVE</strong>(<em>packageName</em>, <em>packageSubpath</em>, <em>parentURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>parentURL</em>).</li>
<li>If <em>packageURL</em> is <strong>null</strong>, then
<ol>
<li>Return <strong>undefined</strong>.</li>
</ol>
</li>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>If <em>pjson</em> is <strong>null</strong> or if <em>pjson</em>.<em>exports</em> is <strong>null</strong> or
<strong>undefined</strong>, then
<ol>
<li>Return <strong>undefined</strong>.</li>
</ol>
</li>
<li>If <em>pjson.name</em> is equal to <em>packageName</em>, then
<ol>
<li>Return the result of <strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>,
<em>packageSubpath</em>, <em>pjson.exports</em>, <em>defaultConditions</em>).</li>
</ol>
</li>
<li>Otherwise, return <strong>undefined</strong>.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_EXPORTS_RESOLVE</strong>(<em>packageURL</em>, <em>subpath</em>, <em>exports</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>If <em>exports</em> is an Object with both a key starting with <em>"."</em> and a key not
starting with <em>"."</em>, throw an <em>Invalid Package Configuration</em> error.</li>
<li>If <em>subpath</em> is equal to <em>"."</em>, then
<ol>
<li>Let <em>mainExport</em> be <strong>undefined</strong>.</li>
<li>If <em>exports</em> is a String or Array, or an Object containing no keys
starting with <em>"."</em>, then
<ol>
<li>Set <em>mainExport</em> to <em>exports</em>.</li>
</ol>
</li>
<li>Otherwise if <em>exports</em> is an Object containing a <em>"."</em> property, then
<ol>
<li>Set <em>mainExport</em> to <em>exports</em>[<em>"."</em>].</li>
</ol>
</li>
<li>If <em>mainExport</em> is not <strong>undefined</strong>, then
<ol>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>mainExport</em>, <strong>null</strong>, <strong>false</strong>, <em>conditions</em>).</li>
<li>If <em>resolved</em> is not <strong>null</strong> or <strong>undefined</strong>, return <em>resolved</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Otherwise, if <em>exports</em> is an Object and all keys of <em>exports</em> start with
<em>"."</em>, then
<ol>
<li>Assert: <em>subpath</em> begins with <em>"./"</em>.</li>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>(
<em>subpath</em>, <em>exports</em>, <em>packageURL</em>, <strong>false</strong>, <em>conditions</em>).</li>
<li>If <em>resolved</em> is not <strong>null</strong> or <strong>undefined</strong>, return <em>resolved</em>.</li>
</ol>
</li>
<li>Throw a <em>Package Path Not Exported</em> error.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_IMPORTS_RESOLVE</strong>(<em>specifier</em>, <em>parentURL</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>Assert: <em>specifier</em> begins with <em>"#"</em>.</li>
<li>If <em>specifier</em> is exactly equal to <em>"#"</em> or starts with <em>"#/"</em>, then
<ol>
<li>Throw an <em>Invalid Module Specifier</em> error.</li>
</ol>
</li>
<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>parentURL</em>).</li>
<li>If <em>packageURL</em> is not <strong>null</strong>, then
<ol>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>If <em>pjson.imports</em> is a non-null Object, then
<ol>
<li>Let <em>resolved</em> be the result of
<strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>(
<em>specifier</em>, <em>pjson.imports</em>, <em>packageURL</em>, <strong>true</strong>, <em>conditions</em>).</li>
<li>If <em>resolved</em> is not <strong>null</strong> or <strong>undefined</strong>, return <em>resolved</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Throw a <em>Package Import Not Defined</em> error.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_IMPORTS_EXPORTS_RESOLVE</strong>(<em>matchKey</em>, <em>matchObj</em>, <em>packageURL</em>,
<em>isImports</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>If <em>matchKey</em> is a key of <em>matchObj</em> and does not contain <em>"*"</em>, then
<ol>
<li>Let <em>target</em> be the value of <em>matchObj</em>[<em>matchKey</em>].</li>
<li>Return the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(<em>packageURL</em>,
<em>target</em>, <strong>null</strong>, <em>isImports</em>, <em>conditions</em>).</li>
</ol>
</li>
<li>Let <em>expansionKeys</em> be the list of keys of <em>matchObj</em> containing only a
single <em>"*"</em>, sorted by the sorting function <strong>PATTERN_KEY_COMPARE</strong>
which orders in descending order of specificity.</li>
<li>For each key <em>expansionKey</em> in <em>expansionKeys</em>, do
<ol>
<li>Let <em>patternBase</em> be the substring of <em>expansionKey</em> up to but excluding
the first <em>"*"</em> character.</li>
<li>If <em>matchKey</em> starts with but is not equal to <em>patternBase</em>, then
<ol>
<li>Let <em>patternTrailer</em> be the substring of <em>expansionKey</em> from the
index after the first <em>"*"</em> character.</li>
<li>If <em>patternTrailer</em> has zero length, or if <em>matchKey</em> ends with
<em>patternTrailer</em> and the length of <em>matchKey</em> is greater than or
equal to the length of <em>expansionKey</em>, then
<ol>
<li>Let <em>target</em> be the value of <em>matchObj</em>[<em>expansionKey</em>].</li>
<li>Let <em>patternMatch</em> be the substring of <em>matchKey</em> starting at the
index of the length of <em>patternBase</em> up to the length of
<em>matchKey</em> minus the length of <em>patternTrailer</em>.</li>
<li>Return the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(<em>packageURL</em>,
<em>target</em>, <em>patternMatch</em>, <em>isImports</em>, <em>conditions</em>).</li>
</ol>
</li>
</ol>
</li>
</ol>
</li>
<li>Return <strong>null</strong>.</li>
</ol>
</blockquote>
<p><strong>PATTERN_KEY_COMPARE</strong>(<em>keyA</em>, <em>keyB</em>)</p>
<blockquote>
<ol>
<li>Assert: <em>keyA</em> contains only a single <em>"*"</em>.</li>
<li>Assert: <em>keyB</em> contains only a single <em>"*"</em>.</li>
<li>Let <em>baseLengthA</em> be the index of <em>"*"</em> in <em>keyA</em>.</li>
<li>Let <em>baseLengthB</em> be the index of <em>"*"</em> in <em>keyB</em>.</li>
<li>If <em>baseLengthA</em> is greater than <em>baseLengthB</em>, return -1.</li>
<li>If <em>baseLengthB</em> is greater than <em>baseLengthA</em>, return 1.</li>
<li>If the length of <em>keyA</em> is greater than the length of <em>keyB</em>, return -1.</li>
<li>If the length of <em>keyB</em> is greater than the length of <em>keyA</em>, return 1.</li>
<li>Return 0.</li>
</ol>
</blockquote>
<p><strong>PACKAGE_TARGET_RESOLVE</strong>(<em>packageURL</em>, <em>target</em>, <em>patternMatch</em>,
<em>isImports</em>, <em>conditions</em>)</p>
<blockquote>
<ol>
<li>If <em>target</em> is a String, then
<ol>
<li>If <em>target</em> does not start with <em>"./"</em>, then
<ol>
<li>If <em>isImports</em> is <strong>false</strong>, or if <em>target</em> starts with <em>"../"</em> or
<em>"/"</em>, or if <em>target</em> is a valid URL, then
<ol>
<li>Throw an <em>Invalid Package Target</em> error.</li>
</ol>
</li>
<li>If <em>patternMatch</em> is a String, then
<ol>
<li>Return <strong>PACKAGE_RESOLVE</strong>(<em>target</em> with every instance of <em>"*"</em>
replaced by <em>patternMatch</em>, <em>packageURL</em> + <em>"/"</em>).</li>
</ol>
</li>
<li>Return <strong>PACKAGE_RESOLVE</strong>(<em>target</em>, <em>packageURL</em> + <em>"/"</em>).</li>
</ol>
</li>
<li>If <em>target</em> split on <em>"/"</em> or <em>"\"</em> contains any <em>""</em>, <em>"."</em>, <em>".."</em>,
or <em>"node_modules"</em> segments after the first <em>"."</em> segment, case
insensitive and including percent encoded variants, throw an <em>Invalid
Package Target</em> error.</li>
<li>Let <em>resolvedTarget</em> be the URL resolution of the concatenation of
<em>packageURL</em> and <em>target</em>.</li>
<li>Assert: <em>packageURL</em> is contained in <em>resolvedTarget</em>.</li>
<li>If <em>patternMatch</em> is <strong>null</strong>, then
<ol>
<li>Return <em>resolvedTarget</em>.</li>
</ol>
</li>
<li>If <em>patternMatch</em> split on <em>"/"</em> or <em>"\"</em> contains any <em>""</em>, <em>"."</em>,
<em>".."</em>, or <em>"node_modules"</em> segments, case insensitive and including
percent encoded variants, throw an <em>Invalid Module Specifier</em> error.</li>
<li>Return the URL resolution of <em>resolvedTarget</em> with every instance of
<em>"*"</em> replaced with <em>patternMatch</em>.</li>
</ol>
</li>
<li>Otherwise, if <em>target</em> is a non-null Object, then
<ol>
<li>If <em>target</em> contains any index property keys, as defined in ECMA-262
<a href="https://tc39.es/ecma262/#integer-index">6.1.7 Array Index</a>, throw an <em>Invalid Package Configuration</em> error.</li>
<li>For each property <em>p</em> of <em>target</em>, in object insertion order as,
<ol>
<li>If <em>p</em> equals <em>"default"</em> or <em>conditions</em> contains an entry for <em>p</em>,
then
<ol>
<li>Let <em>targetValue</em> be the value of the <em>p</em> property in <em>target</em>.</li>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>targetValue</em>, <em>patternMatch</em>, <em>isImports</em>,
<em>conditions</em>).</li>
<li>If <em>resolved</em> is equal to <strong>undefined</strong>, continue the loop.</li>
<li>Return <em>resolved</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Return <strong>undefined</strong>.</li>
</ol>
</li>
<li>Otherwise, if <em>target</em> is an Array, then
<ol>
<li>If _target.length is zero, return <strong>null</strong>.</li>
<li>For each item <em>targetValue</em> in <em>target</em>, do
<ol>
<li>Let <em>resolved</em> be the result of <strong>PACKAGE_TARGET_RESOLVE</strong>(
<em>packageURL</em>, <em>targetValue</em>, <em>patternMatch</em>, <em>isImports</em>,
<em>conditions</em>), continuing the loop on any <em>Invalid Package Target</em>
error.</li>
<li>If <em>resolved</em> is <strong>undefined</strong>, continue the loop.</li>
<li>Return <em>resolved</em>.</li>
</ol>
</li>
<li>Return or throw the last fallback resolution <strong>null</strong> return or error.</li>
</ol>
</li>
<li>Otherwise, if <em>target</em> is <em>null</em>, return <strong>null</strong>.</li>
<li>Otherwise throw an <em>Invalid Package Target</em> error.</li>
</ol>
</blockquote>
<p><strong>ESM_FILE_FORMAT</strong>(<em>url</em>)</p>
<blockquote>
<ol>
<li>Assert: <em>url</em> corresponds to an existing file.</li>
<li>If <em>url</em> ends in <em>".mjs"</em>, then
<ol>
<li>Return <em>"module"</em>.</li>
</ol>
</li>
<li>If <em>url</em> ends in <em>".cjs"</em>, then
<ol>
<li>Return <em>"commonjs"</em>.</li>
</ol>
</li>
<li>If <em>url</em> ends in <em>".json"</em>, then
<ol>
<li>Return <em>"json"</em>.</li>
</ol>
</li>
<li>If <code>--experimental-wasm-modules</code> is enabled and <em>url</em> ends in
<em>".wasm"</em>, then
<ol>
<li>Return <em>"wasm"</em>.</li>
</ol>
</li>
<li>Let <em>packageURL</em> be the result of <strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>url</em>).</li>
<li>Let <em>pjson</em> be the result of <strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>).</li>
<li>Let <em>packageType</em> be <strong>null</strong>.</li>
<li>If <em>pjson?.type</em> is <em>"module"</em> or <em>"commonjs"</em>, then
<ol>
<li>Set <em>packageType</em> to <em>pjson.type</em>.</li>
</ol>
</li>
<li>If <em>url</em> ends in <em>".js"</em>, then
<ol>
<li>If <em>packageType</em> is not <strong>null</strong>, then
<ol>
<li>Return <em>packageType</em>.</li>
</ol>
</li>
<li>If <code>--experimental-detect-module</code> is enabled and the result of
<strong>DETECT_MODULE_SYNTAX</strong>(<em>source</em>) is true, then
<ol>
<li>Return <em>"module"</em>.</li>
</ol>
</li>
<li>Return <em>"commonjs"</em>.</li>
</ol>
</li>
<li>If <em>url</em> does not have any extension, then
<ol>
<li>If <em>packageType</em> is <em>"module"</em> and <code>--experimental-wasm-modules</code> is
enabled and the file at <em>url</em> contains the header for a WebAssembly
module, then
<ol>
<li>Return <em>"wasm"</em>.</li>
</ol>
</li>
<li>If <em>packageType</em> is not <strong>null</strong>, then
<ol>
<li>Return <em>packageType</em>.</li>
</ol>
</li>
<li>If <code>--experimental-detect-module</code> is enabled and the source of
module contains static import or export syntax, then
<ol>
<li>Return <em>"module"</em>.</li>
</ol>
</li>
<li>Return <em>"commonjs"</em>.</li>
</ol>
</li>
<li>Return <strong>undefined</strong> (will throw during load phase).</li>
</ol>
</blockquote>
<p><strong>LOOKUP_PACKAGE_SCOPE</strong>(<em>url</em>)</p>
<blockquote>
<ol>
<li>Let <em>scopeURL</em> be <em>url</em>.</li>
<li>While <em>scopeURL</em> is not the file system root,
<ol>
<li>Set <em>scopeURL</em> to the parent URL of <em>scopeURL</em>.</li>
<li>If <em>scopeURL</em> ends in a <em>"node_modules"</em> path segment, return <strong>null</strong>.</li>
<li>Let <em>pjsonURL</em> be the resolution of <em>"package.json"</em> within
<em>scopeURL</em>.</li>
<li>if the file at <em>pjsonURL</em> exists, then
<ol>
<li>Return <em>scopeURL</em>.</li>
</ol>
</li>
</ol>
</li>
<li>Return <strong>null</strong>.</li>
</ol>
</blockquote>
<p><strong>READ_PACKAGE_JSON</strong>(<em>packageURL</em>)</p>
<blockquote>
<ol>
<li>Let <em>pjsonURL</em> be the resolution of <em>"package.json"</em> within <em>packageURL</em>.</li>
<li>If the file at <em>pjsonURL</em> does not exist, then
<ol>
<li>Return <strong>null</strong>.</li>
</ol>
</li>
<li>If the file at <em>packageURL</em> does not parse as valid JSON, then
<ol>
<li>Throw an <em>Invalid Package Configuration</em> error.</li>
</ol>
</li>
<li>Return the parsed JSON source of the file at <em>pjsonURL</em>.</li>
</ol>
</blockquote>
<p><strong>DETECT_MODULE_SYNTAX</strong>(<em>source</em>)</p>
<blockquote>
<ol>
<li>Parse <em>source</em> as an ECMAScript module.</li>
<li>If the parse is successful, then
<ol>
<li>If <em>source</em> contains top-level <code>await</code>, static <code>import</code> or <code>export</code>
statements, or <code>import.meta</code>, return <strong>true</strong>.</li>
<li>If <em>source</em> contains a top-level lexical declaration (<code>const</code>, <code>let</code>,
or <code>class</code>) of any of the CommonJS wrapper variables (<code>require</code>,
<code>exports</code>, <code>module</code>, <code>__filename</code>, or <code>__dirname</code>) then return <strong>true</strong>.</li>
</ol>
</li>
<li>Else return <strong>false</strong>.</li>
</ol>
</blockquote>
<h4>Customizing ESM specifier resolution algorithm<span><a class="mark" href="#customizing-esm-specifier-resolution-algorithm" id="customizing-esm-specifier-resolution-algorithm">#</a></span><a aria-hidden="true" class="legacy" id="esm_customizing_esm_specifier_resolution_algorithm"></a></h4>
<p><a href="module.html#customization-hooks">Module customization hooks</a> provide a mechanism for customizing the ESM
specifier resolution algorithm. An example that provides CommonJS-style
resolution for ESM specifiers is <a href="https://github.com/nodejs/loaders-test/tree/main/commonjs-extension-resolution-loader">commonjs-extension-resolution-loader</a>.</p>
<!-- Note: The cjs-module-lexer link should be kept in-sync with the deps version --></section>
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